(1) Technical Field
The present invention is generally directed to processes for forming structures incorporating nanoscale objects and more particularly, to selective anchoring and exposure of nanoscale structures within an anchoring structure.
(2) Description of Related Art
(2.1) Nanostructure Properties and Applications
Nanostructures are widely expected to bring about major technological advances. For more than a decade, nanostructures have been the subject of intense study, both in characterization and fabrication. Nanotubes are an integral component in devices requiring reduced power consumption, reduced mass, and extreme functional gains through economies of scale. Unlike larger-scale materials and devices of the same composition, the size-dependent properties of nanoscale devices greatly benefit from their small length scales.
Nanoscale devices are currently only intended for applications in highly-controlled environments. This general lack of robustness in “everyday” environments is a key factor inhibiting the realization of useful nanoscale-based devices. Potentially damaging environmental considerations which bar the use of nanostructures in an environment include the presence of: airborne particles, fluids, impacts and interactions with solid surfaces, and potential interactions with factors such as undesirable temperatures, fluid flows, and chemical reactions. Typical solutions to these environmental hazards involve performing experiments, conducting additional device development, and performing fabrication under highly controlled conditions (usually within a “clean room”). As a result, nanoscale devices are typically restricted to applications in which the devices are in an environment where potentially damaging environmental interactions can be controlled.
A wide variety of useful applications for nanostructures exist. However, lack of viable solutions which mitigate these environmental hazards posed to nano-based technologies typically prohibits them from being used in these applications. A small sample of these applications includes flat-panel displays based on field emissions and hydrodynamic drag-reducing nano- and micro-structured skins and surfaces.
(2.2) Nanostructure Composites
One solution has been to fully immerse the nanostructures within curable materials, thereby forming a composite. Such composites augment mechanical properties such as the elastic modulus and toughness of the nanostructure. In general, this is accomplished by distributing impact forces across the large surface area of the curable material, transferring the force through the composite instead of directly through the nanostructure, and out the bottom surface area of the composite.
Recent developments, specifically in the field of post-fabrication production, have focused on addressing design considerations for fields which have had long felt needs. Most post-fabrication production research has been on developing solutions to the problem of overcoming existing environmental conditions which have historically been unfavorable to nanostructures. As mentioned above, one recent development has focused on immersing nanostructures within curable materials in order to form a nanostructure composite. Composites often consist of a combination of polymeric materials and carbon nanotubes, but the techniques for producing them characteristically lack control of nanotube placement.
One such post-fabrication handling technique uses fixed nanostructures within a deposit of silicon oxide pads via a lithographic shadowmask process onto SiC nanorods and multi-walled carbon nanotubes dispersed on an atomically flat MoS2 surface, leaving some of the nanostructures protruding from the edges of the pads (Wong et al. 1997).
Another method involves dispersing arc-discharge-grown carbon nanotubes within a temperature-cured epoxy resin, curing, and then thinly slicing the composite with a microtome, leaving the nanotubes well-aligned and either fully embedded or tangent to and flush with the slice surfaces (Ajayan et al. 1994).
A further method involves using a blade to smear a UV-curable epoxy over carbon nanotubes deposited onto a surface from a solution, followed by UV curing and mechanical testing (Wagner et al. 1998).
Still another method involves the dispersion of ground arc-discharge grown carbon nanotubes in a thermoplastic polymer, time curing a layer sitting on a Teflon surface, and peeling off the nanotube-polymer composite for mechanical tests (Jin et al. 1998).
Another method uses a chemical vapor deposition growth of aligned arrays of carbon nanotubes followed by submersion of the entire sample and growth substrate within a curable polymer (PMMA or PDMS) solution, followed by curing and removing of the nanotube-polymer composite (Raravikar et al. 2005).
A still further method involves Van der Waals-based attachment of a single carbon nanotube to an etched tungsten tip followed by covering the tungsten tip and the base of the carbon nanotube with a UV-curable polymer using native spreading of the polymer on the tip, then UV curing, leaving the tip “insulated” relative to the surroundings while the nanotube is exposed to the local environment (Boo et al. 2006).
The aforementioned methods for mechanically protecting the nanostructures suffer from unpredictable dispersal of the nanotubes within the materials, generally as illustrated in FIG. 1. In FIG. 1A, a top view of a composite material 100 is shown. The composite material 100 comprises a plurality of nanostructures 104 within a cured material 102. To form the composite material 102, the nanostructures 104 are placed on a layer of fluid material. Spin-coating the material prior to curing results in a largely level surface. The nanostructures 104 become randomly dispersed within the fluid material as a result of the spin-coating process. Although many nanostructures 104 are fully immersed in the cured material 102, many nanostructures 106 randomly protrude from the material 102.
In FIG. 1B, a side view of the composite material 100 is shown. Nanostructures 104 embedded within the cured layer 102 are dispersed randomly. On occasion nanostructures 106 randomly protrude from the cured layer 102. For nanostructures 104 within the cured layer 102, the cured layer 102 offers mechanical protection from impact events. However, for applications which utilize properties in addition to mechanical properties of the nanostructures 104, the nanostructures 106 protruding from surfaces do not protrude with enough regularity and specificity to take advantage of these properties. As such, there is a need for the ability to controllably disperse the nanostructures 104 within a cured layer 102 (polymer) to achieving sufficient wetting, adhesion, and mechanical load transfer between the cured layer and the nanostructures (Xie et al. 2005). There is a further need for the ability to control the depth of a nanostructure 104 within the cured layer 102.
Nanostructure composite 100 manufacturing techniques, as they exist in the art, lack the ability to control the orientation and depth of the nanostructures 104 within the cured layer 102. Therefore, applications requiring anchored partially-immersed nanostructures 106 within a cured layer, such as growth templates for cell and tissue cultures, are unable to take advantage of enhanced characteristics offered by nanostructure composites 100.
Other works have dealt with related topics. None have shown controllable anchoring of wholly immersed nanostructures 104 or partially immersed nanostructures 106 having free ends in exact configurations. For example, in the work of Lahiff et al. 2003, the authors describe that “a thin-film of polydimethylsiloxane (PDMS) was spin coated onto the nanotube film.” Surprisingly the authors further disclose “ . . . which indicates that it is possible only the very tips of the tubes that project from the PDMS surface.” The projection of the exposed nanotube tips from the surface has not proven controllable. In general, the act of spin coating is an imprecise method with no mechanism for controllably immersing a nanostructure within a fluid, such as PDMS.
In other work, Jung et al. 2006, the authors considered the field emission results from their samples, prepared as in Lahiff et al. 2003. The authors state “Scanning Electron Microscope images (not shown) of our functional devices show that the very few tips that are exposed above the PDMS surface are 2-3 μm long and are separated by distances of similar or larger lengths.” Using their approach, the few tips exposed from the surface of the PDMS could not be shown to be controllable. The authors further failed to provide any quantification of how many tips are exposed, a guarantee of reproducibility, or an ability to adjust the degree of protrusion above the surface.
The above methods unintentionally and uncontrollably result in carbon nanotubes (or, more generally, nanostructures) protruding from the upper surface of their PDMS. As a result, using methods that apply a curable polymer on top of the nanostructures offers no repeatable means of control of the number of nanostructures protruding through the surface or the extent of their protrusion. Even when combining a spin-coating procedure with the above procedure, no control over the resulting length of the protrusion of the nanostructures from the surface of the PDMS is demonstrated. Further, residual polymer is likely to cover all portions of the surface since PDMS has been shown to be highly wetting on carbon nanotubes Barber et al. 2004 (PDMS also easily wets many other materials).
Additionally, spin-coating of a material using a highly viscous fluid, such as curable PDMS, on top of nanostructures will significantly alter the nanostructures' local relative configurations. This unavoidable drawback would prevent the formation of specific (pre-determined) small-scale patterns of nanostructures within the cured material.
The above methods fail to provide for control of the overall pattern of nanostructures embedded within the cured layer. A need also exists for controlling the depth and overall height of a nanostructure protruding from a surface. A further need exists for preserving a preexisting pattern of nanostructures within, and optionally exposed or protruding from, the cured layer. A still further need exists for a method that provides a mass-produced composite formed with repeating patterns of nanostructures. A still further need exists for a method that provides the ability to selectively embed specific nanostructures from a group of nanostructures into a composite.